The trend is that, in the future, battery systems will increasingly be used both in stationary applications, for example wind turbines, and in vehicles, such as hybrid and electric vehicles. In order to be able to meet the requirements placed on voltage and available power as set for a respective application, it is known practice for a high number of battery cells to be connected in series. Since the current provided by such a battery needs to flow through all of the battery cells and a battery cell can only conduct a limited current, additional battery cells are often connected in parallel.
FIG. 1 shows a known drive system 100, as is used in electric and hybrid vehicles. The drive system 100 comprises an electric motor M which is connected via three phases L1, L2, L3, also known as conductors or outer conductors, to a pulse-controlled inverter 102. The pulse-controlled inverter 102 comprises a plurality of semiconductor switching elements and is distinguished in that it can convert a DC input voltage Ue into a three-phase output voltage which is present between the three phases L1, L2, L3. A DC-voltage intermediate circuit 104 is connected upstream of the pulse-controlled inverter 102, which DC-voltage intermediate circuit buffers, inter alia, the energy peaks which are briefly necessary, for example in the case of a starting process of the electric motor M. A battery 108 having a plurality of series-connected battery cells 106 is connected to the DC-voltage intermediate circuit 104 via a charging and disconnecting device 110. The charging and disconnecting device 110 comprises switching elements at the plus and minus pole of the battery 108 in order to disconnect said battery from the rest of the circuit, for example in the event of a fault.
Furthermore, a method for controlling a battery 200 is known from DE 10 2011 075 376. As shown in FIG. 2, the battery 200 consists of battery module strings 202, 204, 206 which in each case comprise a plurality of battery modules 208 which are coupled to one another. Each battery module 208 comprises a plurality of series-connected battery cells which are connected to a coupling unit. According to the known method, the coupling unit can couple the battery modules 208 to one another in selectable polarity and, moreover, acts as a pulse-controlled inverter. The battery module strings 202, 204, 206 shown in FIG. 2 correspond to three phases and supply an electric motor M with balanced power in a conventional manner, that is to say each phase supplies a third of the total power. Homogeneous states of charge between the individual battery cells or battery modules are necessary for this purpose in conventional battery systems of this type since the total power is dependent on the weakest battery module.